1. Field of the Invention
The present invention relates to a developing solution which can develop a photosensitive resin composition (referred to as a “photoresist”) on a substrate containing a conductive polymer, with high sensitivity at high resolution, and to a pattern forming method employing the developing solution. The present invention has characteristic features causing no damages to the conductive polymer in contact with the developing solution, and thus can provide a conductive member having low surface resistance and high conductivity in all application fields with which conductive polymers and photoresists are associated.
2. Background Art
Recent years, a material containing indium oxide and tin oxide components (commonly abbreviated as “ITO”) is used as a transparent conductive film material, however, various inorganic and organic materials have been actively studied as alternatives to ITO since indium is a rare element. A conductive polymer, which is an organic material, has a remarkably improved conductivity, and thus is promising as an alternative to ITO.
Conductive polymers have conductivity, translucency and light emitting property, and have characteristic feature of their flexibility higher than that of ITO also after film formation, and have been studied on applications, for example, to transparent conductive films, electrolytic capacitors, antistatic films, batteries and organic EL displays. Some of the conductive polymers have already been put to practical use.
For example, an electronic paper, which is a display element, is required to have flexibility, and a conductive polymer has been studied as a transparent conductive film material.
For an electrolytic capacitor, an attempt has been made to use a conductive solid such as a charge transfer complex and polythiophene in place of conventional electrolytic solution. A conductive polymer having more excellent conductivity can be used to fabricate an electrolytic capacitor having good frequency characteristic. Conductive polymers intended for electrolytic capacitors are also required to be stable chemically and physically and to have excellent heat resistance.
When a conductive polymer is used to form a thin film on the surface of a polymer film or the like, static electricity can be prevented while keeping transparency. Therefore, a coated film is used as a convenient antistatic film, an antistatic container and the like.
The conductive polymer is used as a positive electrode of a secondary battery, for example, in lithium polyaniline batteries and lithium ion polymer batteries.
On the other hand, a conductive polymer can be used, in place of platinum, as a counter electrode of titanium dioxide in dye sensitized solar batteries which are expected as solar batteries cheaper than currently mainstream silicon-based solar batteries. In addition, applications of a conductive polymer to an electronic element such as a diode and a transistor have also been discussed.
Further, there is an organic EL display employing a conductive polymer in its light emitting layer. A flexible display can be produced using an organic material, not glass, as a substrate. The conductive polymer can also be used in a hole transport layer of an organic EL display. The organic EL display is a self-emitting display, and can realize a light-weight thin display having a wide view angle and a high response speed, and thus is now being actively developed as a potential flat panel display.
In this manner, the conductive polymer is an important material for future electronics industry. When the conductive polymer is used, indispensably requires the technique of ensuring the formation of a fine pattern as is the case with ITO.
Fields that require conductive polymer pattern forming involve, for example, electrodes themselves of touch panels, electronic papers, organic EL displays (including low-molecular organic EL displays and high-molecular organic EL displays), inorganic EL displays and self-emitting displays abbreviated as “FED (SED),” and leader lines of the electrodes.
Several methods for the pattern formation of a conductive polymer are known. Patent Document 1 discloses a screen printing method and a printing method utilizing an inkjet or the like. The printing method has the problems of poor pattern accuracy and surface smoothness, although the production steps are simple since film formation is carried out simultaneously with pattern formation. Further, while the formation of a conductive polymer into an ink is necessary for the application of the printing method, there is also the problem of the difficulty in forming the conductive polymer into an ink because the conductive polymer is easy to aggregate.
The photolithographic method is a method for the pattern formation of a conductive polymer by forming a uniform conductive polymer film on the surface of a base material, and then forming a patterned resist film portion (referred to as “resist pattern”) or etching a desired portion of the conductive polymer using a resist pattern as a mask. The photolithographic method involves more steps than the printing method, but provides high pattern accuracy, and thus is a common technique widely used in electron/semiconductor fields. In the photolithographic method, a developing solution and a conductive polymer are brought into contact with each other during the course of the pattern formation of the conductive polymer.
A method for forming the pattern of a conductive polymer by the photolithographic method is disclosed in Patent Document 2. Patent Document 2 discloses a method of forming a pattern of a conductive polymer (referred to as “conductive pattern”) in which a resist pattern is directly formed on a conductive polymer and the conductive polymer is subjected to etching. The document indicates an electron beam resist and a photoresist as usable resists, but does not explain in detail a developing solution necessary to form a resist pattern. As the developing solution, there is only an example employing “MF-312” (manufactured by Shipley Japan) in the Examples. Patent Document 3 indicates that this product “MF-312” is a metal-free developing solution which contains an aqueous solution of tetramethylammonoium hydroxide (hereinafter, referred to as “TMAH”). Although the technique of fabricating a conductive pattern using a conductive polymer by the conductive polymer photolithographic method was known, as described above, conventionally and commonly used developing solutions, such as TMAH, alone were known as developing solutions for use in the development of a resist.
Patent Document 4 discloses a problem that, when an amine-based removing agent is used to remove a resist film on a conductive polymer, the basic amines, ammonia and piperazine contained in the removing agent increase the surface resistance of the conductive polymer, or penetrate into the conductive polymer to reduce the adhesion between a substrate and the conductive polymer. No case of discussions on influences of a developing solution has been found before, in methods for the pattern formation of a conductive polymer using the conventional lithographic method. However, TMAH, conventionally known as a developing solution, is not an amine, but a nitrogen-containing basic substance, and the possibility is conceivable that TMAH, upon contact with a conductive polymer, may worsen the surface resistance of the conductive polymer and the adhesiveness between the conductive polymer and a substrate, as is the case with an amine-based removing agent.
Regarding adverse effects caused by basic amines or the like contained in the removing agent, Patent Document 4 describes, as a solution, the use of a non-amine-based organic solvent as the removing agent. The removing agent is originally intended to non-selectively and rapidly remove a resist resin entirely, and thus was not a technique which can be applied as a developing solution in the developing process of selectively removing a resist by the photolithographic method.
Namely, there was no case of discussions on adverse effects of conventionally known developing solutions on conductive polymers in techniques of forming the patterns of the conductive polymers, nor was a solution therefor unknown.